Add flag for enabling/disabling video fullscreen detection

This CL addes flag for enabling/disabling video fullscreen detection. So
we can reduce the overhead of MediaCustomControlsFullscreenDetector if
the client side doesn't need it.

The downstream change will be in the subclass of AppHooks.java.

BUG=699565

Review-Url: https://codereview.chromium.org/2738893002
Cr-Commit-Position: refs/heads/master@{#457412}
15 files changed